This invention relates to a synchronous motor for use in an air conditioner and a hot water supplier, and to a method for controlling the synchronous motor.
Synchronous motors have been used recently as fan motors for use in air conditioners and hot water suppliers since they can be controlled over a wide range of speeds, their energy consumption is economical, and they can be operated with low acoustic noise. Such a synchronous motor is usually driven by sinusoidal waves generated by a motor drive circuit installed in the housing of the synchronous motor.
Usually, a synchronous motor is provided with Hall ICs, which are inexpensive and simple in configuration, used as position sensors and an inverter main circuit. The Hall ICs detect the positions of the magnetic poles of the synchronous motor. The switching elements in the inverter main circuit are controlled on the basis of the information on the positions of the magnetic poles, and the synchronous motor is driven by the voltages applied to the windings of the motor as a result of the control of the switching elements. Communication between the synchronous motor and the external circuit is usually made by using a DC power source, a control voltage of about 15V, a GND voltage, a DUTY command, and a rotational speed pulse signal, all these items being handled as analog information. The DC power source, the control voltage, the GND voltage and the DUTY command are supplied from the external of the motor to the internal thereof. The rotational sped pulse signal is supplied from the internal of the motor to the external thereof. The DUTY command is a command for controlling the duty ratios of the ON/OFF signals which turn on or off the switching elements on the positive and negative sides of the respective phases of the inverter main circuit. The greater is the DUTY command, the higher is the voltage outputted from the inverter main circuit. Of the items supplied from the external of the motor to the internal thereof, the DUTY command may be used as a torque command and a speed command. In order to isolate the internal of the motor from the external thereof, the DUTY command and the rotational speed pulse signal are fed to the microcomputer external to the motor, via an insulating circuit which serves.
JP-A-2006-34086 discloses a conventional apparatus for driving a synchronous motor.
The conventional apparatus is energized by a DC power source. The current flowing from the DC power source into the motor is detected, and the current signal corresponding to the peak of the detected current is generated. Depending on the current signal and the externally inputted torque command signal (i.e. command signal for current value), the difference signal representative of the value equal to the torque command signal minus the current signal, is outputted. The position sensors deliver the position sensor signals having a constant phase relationship with respect to the voltages induced in the stator windings of plural phases in the motor. Sinusoidal wave signals are generated whose voltage values change periodically depending on the phases of the position sensor signals. The amplitudes of the sinusoidal wave signals are determined on the basis of the difference signal.
On the other hand, the phases of the phase currents flowing through the motor windings are detected, the phase difference between the position sensor signals and the current phase signals is detected, and the phases of the sinusoidal wave signals generated corresponding to the phases of the position sensor signals are corrected depending on the phase difference signal.
On the basis of the amplitudes and the phases of the thus obtained sinusoidal wave signals, the drive signal is generated through the well-known PWM control and then supplied to the gate drive circuit to drive the switching elements in the inverter.
Namely, according to the conventional synchronous motor driving apparatus disclosed in JP-A-2006-34086, the amplitudes of the voltages applied to the motor are determined in accordance with the difference between the current signal corresponding to the peak of the power source current and the externally inputted torque command. Then, in order to drive the motor, the phases of the voltages applied to the motor are determined in accordance with the difference in phase between the position sensor signals and the phase currents.